Internal combustion engines, such as diesel engines, gasoline-fueled engines, and other lean-burn engines, may be operated at higher than stoichiometric air-to-fuel mass ratios. Such engines generally include several pistons, each disposed within a respective cylinder, into which air and fuel are sequentially introduced for combustion. Combustion produces an exhaust gas which may be continually expelled from the internal combustion engine through an exhaust manifold to an exhaust conduit for eventual discharge to the ambient environment.
Because such engines may be operated at higher than stoichiometric air-to-fuel mass ratios, the resulting exhaust gas may include a comparatively higher quantity of oxygen, water, and nitrogen oxides (NOx), e.g., nitric oxide (NO) and/or nitrogen dioxide (NO2), than the exhaust gas of an internal combustion engine operated at another air-to-fuel mass ratio.